BAIKONUR, Kazakhstan — Two U.S. astronauts are set to blast off this week for an expedition that will see the last ever visit to the international space station by a shuttle. But they refused to be daunted by the prospect Monday and said they looked forward to being onboard a fully operational space laboratory.

U.S. astronauts Douglas Wheelock and Shannon Walker and Russia's Fyodor Yurchikhin are set to travel Wednesday onboard a Russian-made Soyuz space craft to the international orbiting laboratory for a six-month mission.

The trio will be onboard the space station to see the final shuttle — the Endeavour — depart from its last planned mission to the lab in November before the fleet is finally retired.

"Of course, it's a big change in our program ... but change is not always bad," said Wheelock, who takes over as commander of Expedition 25 as soon as the current crew returns to Earth in about three months time.

With the shuttle being phased out, the venerable Soyuz rocket will take over as the only means by which astronauts will be able to travel to the space station, which has raised some concern about over-reliance on the Soviet-designed craft.

Wheelock conceded that there was some sadness in seeing the shuttle go, but described his upcoming mission as an exciting prospect.

"It's actually bittersweet to see the shuttle go, but it's really an exciting time as well, because we are also going to be the first to really make full utilization of the space station as a working laboratory," Wheelock said.

He acknowledged that there had been delays and overruns in expenditure in assembling the space station, but he said the wait was fully worthwhile.

"We had big dreams and visions, and I think now we're just getting to that point that we're really going to start seeing a return on that investment," said Wheelock.

He said he was particularly excited about the space station's contributions to the engineering of new materials and its role in ensuring breakthroughs in medicine.

Wheelock, a U.S. Army colonel, is returning to the space station for the first time since his two-week stint on the Discovery in late 2007, when he and his colleagues earned plaudits for their work repairing a power generation facility.

Walker is making her maiden trip to the space station, and thereby following in the footsteps of husband Andrew Thomas, one of a handful of U.S. astronauts to live onboard the old Russian Mir station in the 1990s.

Earlier Monday, the three astronauts received final clearance from a Russian government commission for their mission to go ahead.

Astronauts at Russian-leased Baikonur space center in southern Kazakhstan are kept in strict isolation in the days ahead of any launch to avoid exposure to infection from disease. Wheelock and Walker stood behind a plate of protective glass as they expressed their gratitude in Russian to the technicians that put together the rocket.

The three-person crew will join Russian commander Alexander Skvortskov, NASA flight engineer Tracy Caldwell Dyson and Russia's Mikhail Kornienko, who have been on the orbiting laboratory since April.

Last week, the international space station raised its orbit by 1.6 miles (2.5 kilometers) to enable optimal positioning for the arriving Soyuz craft.

A Progress cargo carrier also is due to arrive at the orbiting lab later this month.

The 12 orbiting astronauts on the International Space Station were hailed by the chief on Thursday — and also got a humorous scolding for keeping their android crewmate locked up in its packing for so long.